1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a method and system for providing adaptive current control timing and responsive current control for interfacing with a dimmer.
2. Description of the Related Art
Electronic systems utilize dimmers to modify output power delivered to a load. For example, in a lighting system, dimmers provide an input signal to a lighting system, and the load includes one or more light sources such as one or more light emitting diodes (LEDs) or one or more fluorescent light sources. Dimmers can also be used to modify power delivered to other types of loads, such as one or more motors or one or more portable power sources. The input signal represents a dimming level that causes the lighting system to adjust power delivered to a lamp, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp. Many different types of dimmers exist. In general, dimmers use a digital or analog coded dimming signal that indicates a desired dimming level. For example, some analog based dimmers utilize a triode for alternating current (“triac”) device to modulate a phase angle of each cycle of an alternating current (“AC”) supply voltage. “Modulating the phase angle” of the supply voltage is also commonly referred to as “chopping” or “phase cutting” the supply voltage. Phase cutting the supply voltage causes the voltage supplied to a lighting system to rapidly turn “ON” and “OFF” thereby controlling the average power delivered to the lighting system.
FIG. 1 depicts a lighting system 100 that includes a leading edge, triac-based dimmer 102. FIG. 2 depicts ideal, exemplary voltage graphs 200 associated with the lighting system 100. Referring to FIGS. 1 and 2, the lighting system 100 receives an AC supply voltage VSUPPLY from voltage supply 104. The supply voltage VSUPPLY, indicated by voltage waveform 202, is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. A leading edge dimmer phase cuts leading edges, such as leading edges 204 and 206, of each half cycle of supply voltage VSUPPLY. Since each half cycle of supply voltage VSUPPLY is 180 degrees of the supply voltage VSUPPLY, a leading edge dimmer phase cuts the supply voltage VSUPPLY at an angle greater than 0 degrees and less than 180 degrees. Generally, the voltage phase cutting range of a leading edge dimmer 102 is approximately 10 degrees to 170 degrees. The leading edge dimmer 102 can be any type of leading edge dimmer such as a triac-based leading edge dimmer available from Lutron Electronics, Inc. of Coopersberg, Pa. (“Lutron”). A triac-based leading edge dimmer is described in the Background section of U.S. patent application Ser. No. 12/858,164, entitled Dimmer Output Emulation, filed on Aug. 17, 2010, and inventor John L. Melanson.
Triac 106 acts as voltage-driven switch, and a gate terminal 108 of triac 106 controls current flow between the first terminal 110 and the second terminal 112. A gate voltage VG on the gate terminal 108 will cause the triac 106 to turn ON and current iDIM when the gate voltage VG reaches a firing threshold voltage value VF and a voltage potential exists across the first and second terminals 110 and 112. The dimmer output voltage Vφ—DIM is zero volts from the beginning of each of half cycles 202 and 204 at respective times t0 and t2 until the gate voltage VG reaches the firing threshold voltage value VF. Dimmer output voltage Vφ—DIM represents the output voltage of dimmer 102. During timer period TOFF, the dimmer 102 chops the supply voltage VSUPPLY so that the dimmer output voltage Vφ—DIM remains at zero volts during time period TOFF. At time t1, the gate voltage VG reaches the firing threshold value VF, and triac 106 begins conducting. Once triac 106 turns ON, the dimmer voltage Vφ—DIM tracks the supply voltage VSUPPLY during time period TON. Once triac 106 turns ON, triac 106 continues to conduct current iDIM regardless of the value of the gate voltage VG as long as the current iDIM remains above a holding current value HC. The holding current value HC is a function of the physical characteristics of the triac 106. Once the current iDIM drops below the holding current value HC, i.e. iDIM<HC, triac 106 turns OFF, i.e. stops conducting, until the gate voltage VG again reaches the firing threshold value VF. The holding current value HC is generally low enough so that, ideally, the current iDIM drops below the holding current value HC when the supply voltage VSUPPLY is approximately zero volts near the end of the half cycle 202 at time t2.
The variable resistor 114 in series with the parallel connected resistor 116 and capacitor 118 form a timing circuit 115 to control the time t1 at which the gate voltage VG reaches the firing threshold value VF. Increasing the resistance of variable resistor 114 increases the time TOFF, and decreasing the resistance of variable resistor 114 decreases the time TOFF. The resistance value of the variable resistor 114 effectively sets a dimming value for lamp 122. Diac 119 provides current flow into the gate terminal 108 of triac 106. The dimmer 102 also includes an inductor choke 120 to smooth the dimmer output voltage Vφ—DIM. Triac-based dimmer 102 also includes a capacitor 121 connected across triac 106 and inductor 120 to reduce electro-magnetic interference.
Ideally, modulating the phase angle of the dimmer output voltage Vφ—DIM effectively turns the lamp 122 OFF during time period TOFF and ON during time period TON for each half cycle of the supply voltage VSUPPLY. Thus, ideally, the dimmer 102 effectively controls the average energy supplied to the lamp 122 in accordance with the dimmer output voltage Vφ—DIM.
The lighting system 100 includes a power converter 123 with a resistor, inductor, capacitor (RLC) network 124 to convert the dimmer voltage Vφ—DIM to an approximately constant voltage and, thus, provide an approximately constant current iOUT to the constant current lamp 122 for a given dimmer phase angle. The triac-based dimmer 102 adequately functions in many circumstances. The triac-based dimmer 102 utilizes a “glue” current during the time TOFF to properly charge the timing circuitry. Additionally, electronic dimmers that include controllers, e.g. “smart” dimmers, utilize current during the time TOFF to provide power to the electronic dimmer. Providing the glue current to the dimmer for the time TOFF has conventionally been considered an unavoidable occurrence.